The Deadliest Year in This Decade

This is the worst year for tornadoes -- by several measures -- since 1999, and there's been a gradual, unsteady increase in the number of tornadoes reported since the middle of the 20th century.

I called the National Weather Service, which says that as of today it knows of 69 dead in tornadoes since Jan. 1, compared to 49 up to this point last year, and 38 deaths for all of 2005. It's worth looking around NOAA's Storm Prediction Center site; find it HERE.

Is there a reason? Shifting weather patterns? Shifting population patterns? Global climate change? Clayton Sandell was asked to put together some notes. Here's what he writes:

Climate and weather experts at the National Oceanic and Atmospheric Administration and NASA said today there is zero evidence at this point that global warming is causing more frequent or stronger tornadoes.

While there has been an observed rise in the number of tornadoes since the 1950's, experts say it's simply due to increasing populations in tornado-prone areas along with better detection technology.

â€œWith the advent of radar, people have been seeing them more," says Gavin Schmidt, a climate scientist at NASA's Goddard Institute for Space Studies. "And I think more people are now living in the middle of nowhere (where tornadoes take place).â€?

"The increase is artificial," says David Easterling of NOAA's National Climatic Data Center in North Carolina. "There's been an increase just because of the increase in population density. There really is no discernable trend in tornadoes that we could say anything about."

Scientists currently have a hard time predicting anything about tornado behavior and links to climate change. That's because the climate models they use to make broad future projections do not have the fine resolution needed to calculate how something as complex as a thunderstorm or tornado might behave. So any talk of a connection between tornadoes and global warming is speculation, say climate specialists. For all they know, global warming might even change atmospheric patterns and conditions in such a way that could lead to fewer tornadoes.

There is however, greater agreement in several other areas when it comes to climate change and extreme weather. There is broad agreement, for example, that as the climate has warmed, heavy rainfall events have increased around the world.

"If you warm the atmosphere up, the atmosphere can hold more moisture," says Easterling, "which then is available to rain out as heavy rainfall event. So it's a signature that's consistent with the notion of global warming."

And there is growing agreement among scientists that tropical storms and hurricanes are getting stronger as the climate warms.

In a report released earlier this year, the Intergovernmental Panel on Climate Change said "it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical sea surface temperatures."

Here again, there is some speculation that as global warming increases, it may at some point produce the type of winds that could break up hurricanes.